1. Field of the Invention
The present invention pertains to a single-crystal growing method suitable for semiconductor materials such as gallium arsenide or silicon and for various metals, utilizing a Czochralski process, and in particular, to an improvement in preventing the fluctuation of crystal diameter during the growing operation.
2. Prior Art
The Czochralski process comprises immersing a seed in a melt contained in a crucible and pulling it up to grow a single crystal in the form of a cylindrical rod. In this method, the configuration of a straight body portion of the single crystal must be controlled precisely in order to improve the characteristics and yield of crystals.
For the configuration control of single crystals of, for example, silicon, optical methods such as a fusion ring detection method and a light reflection method have hitherto been utilized. More specifically, the position of the periphery of the solid-liquid interface is optically detected, and feedback controls of pulling speed, heater temperature, rotational speeds of a pulling shaft and a lower shaft which holds the crucible, and so on, are carried out based on the detected position to thereby maintain the diameter of the straight body portion of the single crystal at a constant value.
However, in the manufacture of single crystals of compound semiconductor such as gallium arsenide (GaAs) or gallium phosphide (GaP), a Liquid Encapsulated Czochralski (LEC) method which involves covering the surface of the melt in the crucible with a liquid encapsulant such as B.sub.2 O.sub.3 in order to prevent the dissociative component from escaping from the melt has been extensively used. In this method, the aforesaid optical methods cannot be employed because of the turbidity of the encapsulant and of flickering due to turbulent high pressure gas. Therefore, there has been developed a television image method, which uses an X-ray transmission device to observe the configuration of the solid-liquid interface. However, this method is not suitable for mass production and is dangerous due to the use of X-rays. There has been also developed a weighing method, which involves estimating the crystal diameter based on a crystal weight measured by load sensors attached to one or both of the pulling shaft and lower shaft of the growing apparatus. However, this method has the disadvantage of being inaccurate due to the sensitivity limit of the load sensor.
Accordingly, there is developed a coracle-type pulling technique as a diameter control method suitable for the LEC method, as disclosed in Japanese Patent Application Laid-Open (18-Month Publication) No. 51-64482. In this technique, a coracle which is heavier than the encapsulant but lighter than the melt is floated on the melt surface, and a single crystal is pulled through a circular opening of the coracle, so that the diameter of crystals can be controlled with high precision.
Furthermore, the aforesaid LEC method has been modified to a Liquid Encapsulated Kyropoulos (LEK) method, which is similar to the LEC method in that B.sub.2 O.sub.3 is floated on the GaAs melt in the crucible. However, in this method, instead of pulling a single crystal through the B.sub.2 O.sub.3 layer, the temperature of the melt in the crucible is gradually decreased to cause a single crystal to grow downward from the seed immersed in the upper portion of the melt, so that the crystal does not pass through the B.sub.2 O.sub.3 layer. In this operation, the seed is pulled up only by an amount which meets the increase of volume during solidification, and hence the single crystal is prevented from contacting the crucible wall.
With this technique, the single crystal is not caused to pass through the B.sub.2 O.sub.3 layer which causes a large temperature gradient, and therefore dislocations, which may be caused due to thermal stresses while passing through the B.sub.2 O.sub.3 layer, are prevented from developing.
However, the aforesaid crystal growing methods have the following disadvantages.
In the method which detects the diameter of single crystals by detecting means using light, X-ray or weighing means and carries out feedback controls, the precision is limited because of the sensitivity limit of the detecting means and the difficulty in optimizing the feedback conditions. Therefore, irregularities of crystal diameter, or under some growth conditions even eccentric crystal shapes cannot be avoided as illustrated in FIG. 1, and hence a single crystal T having a completely cylindrical straight body portion cannot be manufactured.
Furthermore, in the Czochralski method, it is known that the defects of crystals can be reduced if the temperature gradient in the direction perpendicular to the solid-liquid interface is reduced during the crystal growth operation. The pulling speed must be increased to obtain a high yield. However, since the temperature gradient is kept small for an increased pulling speed, heat cannot be sufficiently dissipated from the single crystal. Hence, the problem of the poor configuration control becomes crucial.
In the coracle-type method described above, the diameter control can be achieved with high precision, but the temperature distribution in the melt fluctuates due to the motion of the floating coracle. Therefore, when growing a GaAs (100) single crystal, which has become important in recent years, twinning tends to be caused. Accordingly, this method has been applied industrially only to the manufacture of (111) single crystals. In addition, the coracle floating on the melt may cause some contamination.
There is proposed a method in which a specific member is used to prevent motion of the floating coracle. However, the apparatus is complicated in structure, so that the method cannot be suitably practiced on an industrial basis. In addition, the possibility of contamination further increases due to the use of the member.
Moreover, in the aforesaid LEK method, the growing single crystal cannot be observed since the growth occurs under the B.sub.2 O.sub.3 encapsulant. As a result, it is considerably difficult to grow the crystal so as not to contact the crucible wall. If the crystal should contact the wall, twin crystals or polycrystals may be formed thereat, or the crucible may be damaged. Therefore, it is difficult to practice the method on an industrial basis.